Add styleclass on a button when it is Enable/Disable

We have two different styleclass(commandExButton, commandExButtonDisabled). "commandExButton" class applies for all button. I wanted to apply "commandExButtonDisabled" class only when the button is disable.

Now all the time(Enable/Disable) the letters of the button are grey because the class "commandExButtonDisabled" does not remove when the button becomes Enable. . I am not sure how to remove commandExButtonDisabled class to disaplay the letters of the button white(commandExButton). I am using jquery for that. I would appreciate if someone can help me on this as I am new in jquery. Thank you very much.